1. Field of the Invention
This invention relates in general to the evaluation of integrated circuits, and is specifically related to the method and apparatus for measuring node voltage on an integrated circuit.
2. Description of the Related Art
As the demand for and complexity of integrated circuits increases, it becomes necessary to find more efficient and less expensive ways to evaluate integrated circuits. Measuring internal node voltages is an integral part of evaluating integrated circuits. One conventional method of measuring node voltages is with the use of needle probles. Needle probes are low capacitance probes which are set down on the integrated circuit such that each needle probe physically contacts a node of integrated circuit which is to be measured. However, because the needle probes need to actually contact a node to measure its voltage, the metal line which forms the node may become damaged by contact with the probe. Furthermore, the probe itself will load the node and result in a distorted reading.
Another conventional method of measuring internal node voltages is Electron Beam Testing. Accurate Electron Beam Testers require an electron beam which is usually produced by a Scanning Electron Microscope (SEM) in a vacuum. In operation, the electron beam is focused onto a node of the integrated circuit thereby creating low energy secondary electrons. The energy of the secondary electrons is modulated by the surface potential of the sample. A positive voltage on the node attracts the secondary electrons. Consequently, the electrons slow down. A negative voltage on the node repels the electrons and consequently, the electrons speed up. Usually a specialized secondary electron detector is require to measure the average energy of the secondary electrons. Low average energy indicates slow moving electrons and a positive voltage on the node. On the other hand, a high average energy indicates fast moving electrons and a negative voltage. Accordingly, the relative voltage of the node can be determined by the level of average energy.
There are, however, several undesirable limitations associated with Electron Beam Testing. First, measurements must be taken in a vacuum as air molecules impede the measurement. Second, an Electron Beam Tester requires a significant amount of hardware, especially that required to maintain a vacuum. Accordingly, an Electron Beam Tester is expensive. Finally, Electron Beam Testing is only sensitive to approximately 10-30 mv. Sensitivity to millivolts or even microvolts is desirable.